--- a/devtools/client/framework/toolbox-host-manager.js
+++ b/devtools/client/framework/toolbox-host-manager.js
@@ -39,16 +39,21 @@ let ID_COUNTER = 1;
 
 function ToolboxHostManager(target, hostType, hostOptions) {
   this.target = target;
 
   this.frameId = ID_COUNTER++;
 
   if (!hostType) {
     hostType = Services.prefs.getCharPref(LAST_HOST);
+    if (!Hosts[hostType]) {
+      // If the preferences value is unexpected, restore to the default value.
+      Services.prefs.clearUserPref(LAST_HOST);
+      hostType = Services.prefs.getCharPref(LAST_HOST);
+    }
   }
   this.host = this.createHost(hostType, hostOptions);
   this.hostType = hostType;
 }
 
 ToolboxHostManager.prototype = {
   async create(toolId) {
     await this.host.create();